Question

Which form of xylene has only one monobromo derivative? (1) ortho- (2) para- (3) meta- (4) None of these

Short answer

Para-xylene (Option 2) has only one monobromo derivative.

Step-by-step solution

Understanding Xylene Isomers

Xylene has three isomers: ortho-xylene, meta-xylene, and para-xylene. The difference between these isomers lies in the positions of the two methyl groups on the benzene ring. In ortho-xylene, the methyl groups are adjacent (1,2-), in meta-xylene they are separated by one carbon (1,3-), and in para-xylene they are opposite each other (1,4-).

Identifying Monobromo Derivatives

A monobromo derivative is formed when a single hydrogen atom in the xylene molecule is replaced by a bromine atom. The uniqueness of the derivative depends on the symmetry of the xylene isomer.

Analyzing Ortho-xylene

In ortho-xylene, substituting one hydrogen atom with a bromine atom can occur at multiple positions, leading to non-equivalent compounds. This results in more than one isomer.

Analyzing Meta-xylene

In meta-xylene, multiple unique positions for bromine substitution are available. Hence, it can form more than one monobromo derivative.

Analyzing Para-xylene

Para-xylene is symmetric along the axis passing between the two methyl groups. Any substitution of a hydrogen atom by a bromine atom will produce the same compound. Therefore, para-xylene has only one monobromo derivative.

Conclusion

By analyzing the symmetry and possible substitution positions in each isomer, it is determined that para-xylene (Option 2) is the only isomer among the three that forms a single monobromo derivative.

Key concepts

Monobromo Derivative

A monobromo derivative is an organic compound where a single hydrogen atom is replaced by a bromine atom. This term is particularly important in understanding the different reactions and resultant compounds in organic chemistry.
For xylene isomers, creating a monobromo derivative means substituting one hydrogen in the xylene molecule with a bromine atom.
When studying xylene isomers, you'll notice that:

• Ortho-xylene has adjacent methyl groups (1,2-).
  This allows several non-equivalent positions for bromine substitution, leading to multiple monobromo derivatives.
• Meta-xylene has methyl groups separated by one carbon (1,3-).
  Multiple substitution positions are possible, leading to more than one derivative.
• Para-xylene has opposite methyl groups (1,4-).
  Its symmetry permits only one unique monobromo derivative, regardless of the substitution position.

Understanding monobromo derivatives helps to appreciate the uniqueness and reactivity of different organic compounds.

Symmetry in Organic Chemistry

Symmetry plays a crucial role in organic chemistry, affecting molecular reactivity, physical properties, and the number of possible isomers.
When analyzing xylene isomers, symmetry helps determine the uniqueness of their monobromo derivatives.
For instance:

• Ortho-xylene lacks symmetry between its methyl groups, creating multiple non-equivalent positions for bromine substitution.
• Meta-xylene also lacks symmetry due to one carbon separation between its methyl groups, resulting in multiple substitution possibilities.
• Para-xylene, however, exhibits perfect symmetry along the axis between its methyl groups. This symmetry makes all hydrogen atoms equivalent for substitution, resulting in a single monobromo derivative.

Symmetry simplifies understanding of molecular behavior and reaction pathways.

Isomerism

Isomerism is a fundamental concept in organic chemistry, where compounds share the same molecular formula but differ in structure or spatial arrangement.
Xylene demonstrates three types of structural isomers: ortho-, meta-, and para-xylene.
The key features include:

• Ortho-xylene: Methyl groups are adjacent (1,2-).
  This adjacency affects the compound's reactivity and substitution patterns.
• Meta-xylene: Methyl groups are separated by one carbon (1,3-).
  Separation influences the formation of multiple isomers upon substitution.
• Para-xylene: Methyl groups are opposite each other (1,4-).
  Its unique position and symmetry restrict it to forming only one monobromo derivative.

Isomerism allows for a vast diversity of chemical compounds, each with unique properties and reactions, despite having identical molecular formulas.